In MARK-SPACE digital tank gauging systems, conductor buses directly connected all tank stations with a central control monitoring location. Individual tank stations are in multiplexed circuit connection with the central monitor. During operations, line voltage of the conductor buses must be held at a rather high level, say 48 volts, to avoid accidental tripping of the system by natural phenomenon, such as lightening. In order to create a MARK or SPACE pulse on the lines of interest, the transponder system associated therewith must pull down the MARK or SPACE line to ground potential. Experience has shown that to generate such signals by a separate transponder circuitry formed essentially of MSI's (Medium Scale Integrated Chips) where such circuits are to be formed with minimum device count, simple control capability and low power requirements is exceedingly difficult. This is especially true if the circuit of interest must also be correctly interfaced with present on-site tank gauging systems.
"Red-alert" (i.e., overflow) conditions in refineries and/or chemical complexes require the utilization of a separate end-point transponder system. This has been brought about to a large extent, by changes in Federal and State laws. While normal refining practice seeks to avoid overloading storage tanks in any storage tank network, Federal and State laws have now increased the pentalties attendant such spillage (sometimes irrespective of proximate cause) such that separate end-point monitoring systems are now advisable. However, experiences show that solid state end-point transponder systems are difficult to achieve because of the reasons previously mentioned.